As the cost of energy increases, more pressure is being brought to bear on energy efficiency in connection with the growing of plants in greenhouses or other enclosed and energy-controlled environments. It is now widely recognized in the industry that one of the most efficient ways of growing plants is to create a micro-climate for the plants, primarily by heating the root zone of the plants, rather than to attempting to control the air temperature around the plants. In some installations it has been even found most advantageous to heat the plant root zone while cooling the foliage. Thus, considerable work has been undertaken recently in connection with water-based heating systems which are installed underneath plant containers or in propagation benches or beds in combination with space heating or cooling apparatus.
Root heating or micro-climate temperature control systems have been found to achieve heat energy savings between about 35% and as much as about 78%, as compared to conventional space heating systems. Prior root heating systems, however, have also been accompanied by a significant increase in the system installation costs, as compared to space heaters.
Typical of the root zone heating apparatus which has previously been marketed is the system marketed under the trademark FLEXITWIN by Calmac Manufacturing Corporaction of Inglewood, N.J. This system is shown in a solar panel in U.S. Pat. No. 4,112,921 but it has also been used as a microclimate heat transfer array for root zone heating. The Calmac system employs pairs of polyethylene tubes, which in fact are not very flexible and which are connected to input and outlet headers for the flow of water at an elevated temperature through the tubes. The tubes are extruded together as a unit with abutting side-by-side channels, and a copper or similar U-shaped fitting is mounted and secured in the far end of the tube to provide a continuous loop. A feature of this system is that the counterflow of heated water in the abutting tubes purportedly evens out or averages out the temperature differences so that at any position along the pair of tubes the temperature is essentially the same. U.S. Pat. No. 3,893,507 also shows a counterflow tubular array used to maintain the temperature in a slab of ice.
Another approach in the prior art has been to employ a mat in which there are a plurality of side-by-side plastic tubes interconnected by a webbing and positioned under the plant root system or zone. U.S. Pat. Nos. 4,159,595 and 4,270,596 are typical of such prior art mats. While substantial energy savings can be attained through use of a mat-type root heating system, the initial cost of the system is undesirably high, and they lack flexibility in accommodating spacing variations between tubes as well as being poorly suited for positioning of selected tubes above the mat to augment space heating.
In recent years another consideration has become more important in connection with greenhouse heating. While greenhouses have the advantage of eliminating transportation costs since they can be located in almost any climate proximate large populations, they still have a relatively low priority with regard to the use of conventional energy sources. Thus, oil, gas and liquid petroleum heating units are still in widespread use to heat greenhouses, but there is also considerable pressure to enact laws and codes which would limit or prohibit use of such conventional energy sources as the basis for greenhouse heating systems. Accordingly, the future for greenhouse heating appears to reside in the use of energy sources such as solar energy, solar ponds, geothermal sources, and the like.
Many of these alternate energy sources, however, inherently create new technological problems which have not been addressed or resolved in prior root heating systems. Thus, both solar ponds and geothermal sources produce water at an elevated temperature which includes a very high percentage of corrosive materials. Polyethylene tubing of the type conventionally employed heretofore in greenhouse heating systems is not capable of withstanding the corrosives in geothermal water nor the brine in solar ponds. One way of dealing with the brine from solar ponds or the naturally occurring corrosives in geothermal water is to provide a heat exchanger which is then used to isolate the energy source from the greenhouse heating system. This approach, however, obviously adds to the overall system cost. Copper tube systems will withstand higher temperatures, but they are subject to corrosion, and the cost of such systems is very substantial. High molybdenum content stainless steel will withstand both the temperatures and corrosion, but the cost of providing substantial quantities of tubing out of such a material is prohibitive. U.S. Pat. Nos. 3,470,943 and 3,521,699 are typical of geothermal energy conversion systems.
The efficiency of solar panels diminishes significantly as water temperature increases. Accordingly, relatively large collectors are required to achieve high temperature output water. Thus, a heating system which can employ low temperature solar panel heated water, will effect a significant cost savings in the cost of solar panels.
Another problem which has existed with prior radiant heating systems for plants has been the need to provide auxiliary space heaters for installations in very cold climates. While root zone heating systems are more efficient in producing a unit of plant growth, in northern climates the air temperature in the greenhouse can be so low as to require space heating around the plants, in addition to root zone heating, in order achieve the necessary plant growth. In climates where the outside temperature may be at or below 0.degree. F., for example, some space heating of the greenhouse is required in addition to the root heating.
Most prior systems have used a belt-and-suspenders approach by providing separate space heaters to augment the root heating system. This will, of course, undesirably increase the initial installation cost. Alternatively, rather complex water-gas greenhouse plants and even in open-air applications, see, e.g., U.K. Pat. No. 2,060,341 and U.S. Pat. Nos. 3,727,345 and 3,863,710. In addition to the complexity and inherent high cost of such systems, they lack flexibility in accommodating a wide variety of plant arrays and heating and/or cooling needs.